Paul Giamatti has had many film and stage roles that were physically and mentally taxing. But as the principal star of HBO's seven-part miniseries John Adams (Parts 1 and 2 premiere Sunday, 8 ET/PT), the Oscar-nominated actor got a lot more than he bargained for.

"When I first got the (script), I thought I wasn't going to do it. It was endlessly daunting. There was an enormous about of material to memorize," says Giamatti, a character actor whose career centered mostly on supporting roles until 2003's American Splendor and 2004's Sideways. "But then I thought, 'When is something like this going to come up again?' "

Playing the second president of the United States, Giamatti anchors a production that expanded from seven to 81/2 hours during shooting. "Some stories were so big, it was hard to tell them in 60 minutes," says Adams director Tom Hooper. Based on David McCullough's Pulitzer Prize-winning biography John Adams, "the story felt like a neglected treasure, a brilliant lost history of the American Revolution waiting to be told," he says.

Giamatti portrays Adams from his roots as a Massachusetts farmer and lawyer, as an emerging revolutionary, Founding Father, president and diplomat, aging from his 30s to 90. To prepare for the role, Giamatti read several books about Adams as well as letter exchanges with his wife, Abigail.

"It was an unusually close relationship," he says. Adams' wife of 54 years was a politically savvy confidant and early supporter of women's rights.

In Adams, Giamatti discovered a complicated, often contradictory soul. "I went in thinking Adams was boring. He's anything but," he says.

"I found him fantastically complicated. He was accessible and relatable, but a contradiction — constantly questioning his own motives. He was vain, but he despised himself. He was at war with himself. He was kind of a mess."

Giamatti shaved his head to better accommodate hairpieces and wigs of the time. "Those wigs were a source of great humor on the set," he says. Still, Giamatti, 40, was so stressed during the 110-day shoot in Virginia and Budapest, Hungary, from last February to July, that he said he began smoking again after eight years.

"Everyone is perfectly cast to play these iconic figures," Giamatti says.

Adams co-producer Tom Hanks, who shared screen time with Giamatti in 1998's Saving Private Ryan, says Giamatti was ripe for the role.

"I never thought we'd get someone with his talent who would forgo other opportunities to take this part," Hanks says. "The physical demands of this gig and the amount of verbiage you had to spill out were brutal."

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Successor and predecessor: John Adams (Paul Giamatti, left) and George Washington (David Morse) in the seven-part HBO miniseries. The nation's second president "was at war with himself," Giamatti says. "He was kind of a mess."

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